REPORT OF THE DIRECTOR IQII 159 



abundance of deep green chlorite which forms from both feldspar 

 and hornblende but, so far as the evidence of a single section may 

 be trusted, more from the former than the latter. This chlorite is 

 fibrous and of a strong, deep green color persisting with crossed 

 nicols w^hich bring out aggregate polarization and weak double 

 refraction. Pyrite in large masses is abundant, lying imbedded 

 almost exclusively in the chlorite and molded upon hornblende and 

 apatite instead of replacing them (see figure 24). The pyrite is 

 frequently in slender branching aggregates which have grown in and 

 been supported by the chlorite, and is clearly younger than the 

 latter mineral. 



As bearing upon the mobility of the chlorite, referred to in con- 

 nection with previous sections, an interesting fact appears in this 

 slide. At one point a very narrow crack has developed in a grain of 

 pyrite, crossed an area of chlorite, torn it away from another grain 

 of pyrite, and separated a corner of the latter. The crack throughout 

 its course is sharply defined by narrow black boundaries and is 

 completely filled with a later deposit of chlorite. The latter is the 

 same deep green as the old chlorite and in ordinary light would not 

 be distinguished from it but for the dark boundaries of the crack. 

 With crossed nicols, however, the younger chlorite in the crack shows 

 different optical orientation and so is sharply distinguished from the 

 older mineral (see figure 25). 



With this diagrammatic case before us it is easy to find many simi- 

 lar though less clear instances, and it becomes evident, as suggested 

 previously, that the chlorite is very mobile, circulating relatively 

 freely and thus tending to fill all spaces, however minute, in the rock. 

 It is also clear that the several cases previously described where 

 there seemed to be conflicting evidence as to the relative age of 

 pyrite and chlorite are cleared up by this proof of at least two 

 periods of chlorite formation, one before, the other after, the de- 

 position of pyrite. As to the larger question of the date of pyrite 

 concentration compared with that of the possible intrustion of the 

 hornblendic rocks, an indication is given by the presence of clearly 

 later pyrite masses in these rocks. That the same later date 

 holds good for the larger ore bodies is probable, and this may 

 account for the fact, already stated, that the ore deposits show no 

 marked difference when compared with those not directly associated 

 with similar rocks, but on the contrary agree in all essential features 

 with the prevailing type as developed in the midst of extensive belts 

 of Grenville sediments. 



